1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a method and an apparatus for computing a precise location using differential carrier phases of a GPS satellite signal and a Wide Area Augmentation System (WAAS) or similar satellites.
2. GPS Background
The Global Positioning System (GPS) was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite""s onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the techniques deployed. This invention specifically targets the sub-centimeter accuracies achievable on a remote and possibly mobile GPS receiver by processing carrier phase observations both from the remote receiver and from one or more fixed-position reference stations. This procedure is often referred to as Real-Time-Kinematic or RTK.
To gain a better understanding of the accuracy levels achievable by using the GPS system, it is necessary understand the two types of signals available from the GPS satellites. The first type of signal includes both the Coarse Acquisition (C/A), which modulates the L1 radio signal and precision (P) code, which modulates both the L1 and L2 radio signals. These are pseudorandom digital codes that provide a known pattern that can be compared to the receiver""s version of that pattern. By measuring the time-shift required to align the pseudorandom digital codes, the GPS receiver is able to compute an unambiguous pseudo-range to the satellite. Both the C/A and P codes have a relatively long xe2x80x9cwavelength,xe2x80x9d of about 300 meters (1 microsecond) and 30 meters ({fraction (1/10)} microsecond), respectively. Consequently, use of the C/A code and the P code yield position data only at a relatively coarse level of resolution.
The second type of signal utilized for position determination is the carrier signals. The term xe2x80x9ccarrierxe2x80x9d, as used herein, refers to the dominant spectral component which remains in the radio signal after the spectral content caused by the modulated pseudorandom digital codes (C/A and P) is removed. The L1 and L2 carrier signals have wavelengths of about 19 and 24 centimeters, respectively. The GPS receiver is able to xe2x80x9ctrackxe2x80x9d these carrier signals, and in doing so, make measurements of the carrier phase to a small fraction of a complete wavelength, permitting range measurement to an accuracy of less than a centimeter.
In stand-alone GPS systems that determine a receiver""s position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the satellite""s clock reference, the location of the orbiting satellite, ionospheric induced propagation delay errors, and tropospheric refraction errors. A more detailed discussion of these sources of error is provided in U.S. Pat. No 5,828,336 by Yunck, et al. A large portion of the satellite""s clock error, referred to as Selective Availability (SA) is purposefully induced by the U.S. Department of Defense to limit GPS accuracy to non-authorized users. SA can cause positioning errors exceeding 40 meters, but even without SA, errors caused by the ionosphere can be tens of meters.
To overcome the errors of the stand-alone GPS system, many kinematic positioning applications make use of multiple GPS receivers. A reference receiver located at a reference site having known coordinates receives the satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver""s coordinates relative to the reference receiver""s coordinates.
The technique of differencing signals is known in the art as differential GPS (DGPS). The combination of DGPS with precise measurements of carrier phase leads to position accuracies of less than one centimeter root-mean-squared (centimeter-level positioning). When DGPS positioning utilizing carrier phase is done in real-time while the remote receiver is potentially in motion, it is often referred to as Real-Time Kinematic (RTK) positioning.
One of the difficulties in performing RTK positioning using carrier signals is the existence of an inherent ambiguity that arises because each cycle of the carrier signal looks exactly alike. Therefore, the range measurement based upon carrier phase has an ambiguity equivalent to an integral number of carrier signal wavelengths. Various techniques are used to resolve the ambiguity, which usually involves some form of double-differencing. Some prior art related to this is U.S. Pat. No. 4,170,776 by MacDoran, U.S. Pat. No. 4,667,203 by Counselman, U.S. Pat. No. 4,963,889 by Hatch, U.S. Pat. No. 5,296,861 by Knight, and U.S. Pat. No. 5,519,620 by Talbot et al. Once ambiguities are solved, however, the receiver continues to apply a constant ambiguity correction to a carrier measurement until loss of lock on that carrier signal.
Regardless of the technique deployed, the problem of solving integer ambiguities, in real-time, is always faster and more robust if there are more measurements upon which to discriminate the true integer ambiguities. Robust means that there is less chance of choosing an incorrect set of ambiguities. The degree to which the carrier measurements collectively agree to a common location of the GPS receiver is used as a discriminator in choosing the correct set of ambiguities. The more carrier phase measurements that are available, the more likely it is that the best measure of agreement will correspond to the true (relative to the reference GPS) position of the remote GPS receiver. One method, which effectively gives more measurements, is to use carrier phase measurements on both L1 and L2. The problem though is that it is relatively difficult to track L2 because it is modulated only by P code and United States Department of Defense has limited access to P code modulation by encrypting the P code prior to transmission. Some receivers are capable of applying various cross-correlation techniques to track the P code on L2, but these are usually more expensive receivers that L1 only capable receivers (see, for example U.S. Pat. No. 5,293,170 Lorenz, et al.).
Other approaches have been employed to gain additional measurements on GPS receivers in an attempt to enhance RTK. Hatch, for example in xe2x80x9cPseudolite-aided method for precision kinematic positioningxe2x80x9d (U.S. Pat. No. 5,177,489) suggests the use pseudolites, which due to their proximity exhibit rapid changes in relative location causing the apparent affect of additional satellites. A derivation of this, which uses Low Earth Orbit (LEO) satellites, that travel across the sky much more rapidly than GPS satellites is presented by Enge, et al. in xe2x80x9cMethod and receiver using a low earth orbiting satellite signal to augment the global positioning systemxe2x80x9d (U.S. Pat. No. 5,944,770). The use of the GLObal NAvigation Satellite System (GLONASS) satellites from the former USSR has been spelled out in xe2x80x9cRelative position measuring techniques using both GPS and GLONASS carrier phase measurementsxe2x80x9d by Kozlov, et al. (U.S. Pat. No. 5,914,685).
Nevertheless, it is often desired to perform RTK on low-cost L1 only receivers that do not have access to the GLONASS system, pseudolites, or LEO satellite signals. The present invention describes a new approach of applying Wide Area Augmentation System (WAAS) carrier phase measurements to the solution of the ambiguity problem to give greater speed and reliability of ambiguity solution. Another feature of the present invention is to further use these WAAS measurements to aid in positioning and cycle-slip (loosing track of the integer ambiguity) detection. Yet another feature of the described invention is to extend the operating range of the rover receiver relative to the base receiver unit by using WAAS ionosphere models to remove ionospheric error components that would otherwise prevent or degrade operation as the rover-to-base separation increased. Thus, the present invention exhibits the benefit of improving the overall robustness of the L1-only RTK solution
The WAAS system is an augmentation to the current Global Navigation Satellite System (GNSS) that includes GPS, GLONASS and other satellite ranging technologies. It was conceived as a means to meet the stringent integrity, availability, and accuracy requirements necessary to use GPS as the sole means navigation for civilian aviation. The term WAAS here is used as a generic reference to all GNSS augmentation systems which, to date, include three programs: WAAS (Wide Area Augmentation System) in the USA, EGNOS (European Geostationary Navigation Overlay System) in Europe and MSAS (Multifunctional Transport Satellite Space-based Augmentation System) in Japan. Each of these three systems, which are all compatible, consists of a ground network for observing the GPS constellation, and one or more geostationary satellites.
The role of the ground observation network is to determine, in real-time: the operational performance characteristics of any visible GPS satellites; to compute corrections to the information which comes from those satellites; to upload this information to the geostationary satellites; and to control the operation of these geostationary satellites which effectively augment GPS with differential range corrections and additional integrity information.
The WAAS (WAAS, EGNOS, and MSAS) geostationary satellites broadcast signals on the L1 frequency using only a C/A code with a superimposed navigation message. These signals are similar to L1, C/A code broadcast by GPS satellites except that the WAAS signals are modulated with 250 bit-per-second integrity-related information and GNSS satellite range corrections derived from data received from the observation network. All range corrections are relative to the GPS C/A code only (not carrier or P code). For the intentions of WAAS, this is the preferred approach since C/A code is the most straight-forward signal to demodulate (P codes are also broadcast by GPS but are difficult to demodulate due to encryption) and is the ranging signal provided by the GPS satellites for commercial use.
Although WAAS improves robustness over stand-alone GPS and improves positional accuracy to roughly the one-meter level, it, until this invention, has offered little to the precise positioning needs typical of many users of RTK systems. The reason being is, as discussed above, that WAAS is intended to provide enhancements to GPS positioning systems based around measurements of C/A code. Nothing in the WAAS specification (for example see RTCA/DO-229A) relates to the use of WAAS to enhance RTK. Thus, there was a need for a method to enhance RTK employing the employing the advantages WAAS could provide.
Briefly, a preferred embodiment includes a GPS/WAAS reference receiver that is stationary, a GPS/WAAS rover receiver that may be stationary or may be in motion, and a communication link between them for delivering reference data from the reference receiver to the rover receiver. Both the GPS/WAAS reference receiver and the GPS/WAAS rover receiver determine the carrier phases from GPS satellite signals and at least one WAAS carrier phase from a WAAS satellite signal. Each of the receivers contains a signal processor for processing the data. The GPS/WAAS rover receiver receives an airwave signal that includes reference data for the GPS and the WAAS carrier phases determined at the GPS/WAAS reference receiver and computes a difference between these carrier phases and the carrier phases of the GPS and the WAAS satellite signals determined by the GPS/WAAS rover receiver. The location vector of the rover receiver, relative to the location of the reference receiver, is determined in real-time to centimeter level accuracy by exploiting the GPS and WAAS carrier phase measurements.
The GPS rover receiver computes the location vector from a double or single difference of the GPS rover and reference carrier phases for a plurality of GPS satellites and at least one WAAS satellite. A radio transmits information for the carrier phases, code phases, and the time of measurement determined at the GPS reference receiver to a radio receiver at the GPS rover receiver over a terrestrial link.
According to a more specific aspect of the present invention, in order to solve the integer ambiguity problem, the signal processing is performed in a manner such that the information from the WAAS satellite is exploited to determine the integer number of wavelengths that must be applied to the raw carrier phase difference measurements from both GPS and WAAS satellites. The additional data from the WAAS satellite improves the speed and overall reliability of the process for solving for the correct integer number of wavelengths. The overall result is that the relative position of the rover receiver can be ascertained more quickly and more reliably than without the use of the WAAS satellite.
Another aspect of the invention is the use of the transmitted WAAS ionosphere map to mitigate errors caused by the ionosphere when performing RTK positioning. As the separation between receivers increases, atmospheric effects on the measured pseudoranges and carrier phase observations start to become noticeably different between the rover and reference receiver. Consequently, these errors can no longer be eliminated by the difference. Errors caused by the troposphere can be modeled to some degree, which is especially useful when significant height differences are present between the rover and reference receivers. Prior to the invention described herein, however, there was no convenient or rapid way to determine differences in signal propagation delays induced by the ionosphere.
Another aspect of the invention is the use of WAAS satellites in addition to GPS satellites effectively increases the number of carrier phase measurements that can be used by a receiving system at a particular location. Thus, mitigating difficulties sometimes encountered when one or more GPS satellites are blocked from the view of a receiver by a hill, building, or the like. The concept can be implemented in a number of ways, but stated simply, the more satellites that are tracked, the more likely it is that the tracked satellites will not drop below a critical number necessary to maintain accurate knowledge of the GPS receiver""s position. If the position is known, then cycle slips can be corrected by calculating the true geometric range to the satellite, adjusting for known clock errors and comparing the calculated ranges to the measured range involving the suspected cycle slip. If the difference between measured and calculated range is significant (close to a cycle or more) then adjust the measured range to the nearest integer number of cycles to the difference.
Therefore, the present invention provides a Global Positioning System (GPS) remote user receiver, a GPS reference receiver, both capable of making GPS and WAAS satellite measurements, and a method for combining GPS and WAAS carrier phase signals for accurately finding a location vector between the GPS user receiver and the GPS reference receiver.
The present invention also provides a method of combining GPS and WAAS signals for improving the speed and reliability of resolving a number of wavelengths of a satellite signal from a GPS satellite to the GPS receiver and from a WAAS satellite to the GPS.
Another feature of the present invention to provide a method to use the WAAS ionosphere delay map for modeling errors in differential GPS systems utilizing carrier phase measurements, thereby increasing the geographical area of usefulness of a differential carrier phase measurement.
Yet another feature of the present invention is to provide a method of combining WAAS and GPS signals for improving the process of detecting cycle slips in carrier phase while performing RTK positioning. The additional redundancy of the WAAS carrier phase measurements aiding in detecting cycle slips.